1. Field of the Invention
The present invention relates to a differential signal comparator which handles a differential signal.
2. Description of the Related Art
A high-speed differential interface circuit is used according as signals have been increased in speed in recent years, typified by a reduced swing differential signaling (RSDS) or a low voltage differential signaling (LVDS).
In these circuits, a differential signal is used as an input signal, so that a voltage comparator circuit with a differential input is used in the receiver circuits. The input differential voltage requires a differential signal component of ±50 mV and common mode signal component of so-called rail-to-rail (power supply to GND).
The differential signal comparator which handles such a differential signal is an application of a differential amplifier. A circuit, as an example of a differential amplifier circuit, is disclosed in Japanese Patent Application Laid-Open No. 2006-148364. FIG. 5 illustrates the differential amplifier circuit in the application. The figure includes input differential pairs DF61 and DF62, current mirrors CM61 to CM64 and constant currents I61 and I62.
In the circuit structure illustrated in FIG. 5, when the input signal is in the vicinity of rail-to-rail, for example, in the vicinity of GND, only the input differential pair DF61 operates. For this reason, if the input voltage is:    (1) In+>In− and    (2) In+<In−,with the same constant current I61, in the above case (1), the current mirror CM61 with the ratio of 1:1 is driven, and in the above case (2), the current mirror CM62 with the ratio of 1:k is driven. For this reason, there exists difference in a load between the cases (1) and (2), that is to say, difference in the node parasitic capacitances across the gate portions of the current mirrors CM61 and CM62. The difference in the load makes the response of the current mirror asymmetric, creating a problem in that the current mirror generates a voltage comparison output whose duty ratio is different from the duty ratio of the input signal.